An electric double layer capacitor is constituted by winding or laminating a plurality of conductive foils of aluminum or the like forming a current collector on a separator, and sealing in a case together with a nonaqueous electrolyte solution. This electric double layer capacitor is, recently, expanding its applications in regeneration or electric power storage for various electric appliances and electric vehicles. Accordingly, the electric double layer capacitor is further demanded to be higher in performance, larger in capacity, superior in reliability, and lower in cost.
In the hitherto proposed electric double layer capacitor, as disclosed in Japanese Laid-open Patent No. 1-164017, a binder such as polytetrafluoroethylene (PTFE) is kneaded with activated carbon or carbon to form a current collector. It is, however, difficult to disperse fluorine derivative binder material uniformly in the current collector, and it was another problem to use an exclusive solvent. To solve these problems, in Japanese Laid-open Patent No. 6-316784, the use of ultrasonic homogenizer is proposed as a manufacturing technique of uniform mixed powder of carbon black and PTFE, while Japanese Laid-open Patent No. 6-203849 and Japanese Laid-open Patent No. 8-203536 propose the use of ultrasonic homogenizer as a technique for dispersing catalyst and nickel together with carbon black in the fuel electrode of fuel cell, its catalyst manufacturing method, and battery operating method. In such methods, however, there was a limit in enhancement of dispersion.
Incidentally, Japanese Laid-open Patent No. 63-104316 proposes the use of elastomer of which glass transition temperature is -10 deg. C. or less as the current collector. As examples of the elastomer, binders are proposed such as NBR, SBR, fluororubber, and silicone rubber. Also proposed is a method of dissolving such binder in an organic solvent, mixing Ketienblack therein to disperse the two, evaporating the solvent, and forming by blending with a roll. It is further proposed to wind an elastomer around a roll, add Ketienblack, mix, blend, and form simultaneously. Such techniques are same as the conventional methods used in manufacture of tires and other rubber kneaded products, and it is difficult to manufacture current collectors of high capacity such as lithium secondary battery and lead secondary battery by such methods. As a similar example, Japanese Laid-open Patent No. 7-331201 proposes to knead rubber material as a binder of expanded graphite. In this case, the rubber material is dissolved in a solvent such as toluene, and carbon powder is added and kneaded, and heated. In a method proposed in Japanese Laid-open Patent No. 8-250380, meanwhile, powder of acrylonitrile-butadiene rubber is dissolved in a solvent such as xylene, and mixed with activated carbon powder and acetylene black, and finally the solvent is evaporated, and the obtained mixture is formed in a thickness of 50 to 500 microns by pressure forming method or by using extrusion forming die. In such conventional dissolving methods, since the rubber material is completely dissolved (in other words, dissolved or dispersed to a molecular state of several angstroms), and also pores for forming the electric double layer of several angstroms on the surface of activated carbon are clogged, it was a problem that the product capacity was lowered significantly. To solve such conventional problems, methods of kneading rubber and activated carbon or forming the current collector have been proposed, but there were limits.
Meanwhile, as proposed in Japanese Laid-open Patent No. 62-16506 or Japanese Laid-open Patent No. 62-179711, activated carbon powder is dispersed in latex using water as dispersant, the mixed solution is dehydrated to remove solvent such as dispersant, the selected aggregated matter is dried, this aggregated matter is crushed, granulated, and finally pressurized, and a disk-shaped current collector of 16 mm in diameter and 0.9 mm in thickness is formed. In this case, however, it takes cost in drying and crushing.
On the other hand, as proposed in Japanese Laid-open Patent No. 3-280518, ammonium salt of carboxyethyl cellulose or the like is dissolved in water, and activated carbon is mixed and dispersed, and the prepared solution is applied on an aluminum base material by a technique such as roll coating or doctor blade coating to manufacture an electric double layer capacitor. Besides, Japanese Laid-open Patent No. 57-60828 proposes to enhance the coat film density by a press, but since the current collector is stiff and brittle, the coat film density is not enhanced unless pressed at a high pressure. When pressed at a high pressure, however, the electrode coat film may be broken, or may be peeled off from the conductive foil, and further the conductive electrode may be elongated or deformed like seaweed. In this case, by decreasing the resin amount, the pressing pressure may be somewhat lowered, but since the resin amount is small, the binding density of the current collector coat film is insufficient (for example, the adhesion strength of the conductive foil and activated carbon coat film is insufficient, and aggregation breakage occurs in the activated carbon coat film itself). Therefore, it was a problem that the current collector was broken or peeled off when winding or laminating.